The present invention relates to hybrid networks, and more particularly to an improved network for selectively combining microwave power presented at two input ports of the network at an output port with minimum insertion loss.
Four port hybrid networks for combining at one output port the power generated by two RF sources and presented at two ports of the network are well known in the art. An exemplary reference is the paper "A Method of Analysis of Symmetrical Four-Port Networks," by J. Reed and G. J. Wheeler, IRE Trans. MTT, October 1956, pp. 246-252. In general, these networks are symmetrical and may be implemented in strip-line, coaxial lines, waveguide, or other transmission lines. In a typical configuration, the network may be configured as a 3 decibel (dB) multiple branch hybrid network, wherein the RF power presented at the two input ports is combined at an output port, the fourth port of the network comprising the isolated port. This network configuration may also be viewed in the reciprocal sense as a power divider wherein the RF power presented at one input port is divided between two output ports, the fourth port of the network comprising the isolated port.
One typical application of a hybrid power combiner is in solid state transmit/receive modules for radar systems, in which the outputs of two high power RF transistors are combined by a hybrid network. For a two transistor combiner circuit, if one transistor is turned off, the output power at the output port of the combiner drops, not by 3 dB, but by 6 dB because now the network operates as a power divider circuit wherein the RF power from the remaining operational transistor will be divided between the isolation port and the output port. Thus, the output power of the combiner circuit will be reduced to 25% of the power provided by two operational transistors, even though one of the two power transistors is still operational.
Another typical application of a four-port hybrid power combiner is in solid state transmitters for radar systems, where aperture amplitude distribution control is required. In this application, the outputs of numerous high power RF transistors are combined, utilizing a network comprising a number of four port hybrid networks. For a network combining N transistors, the decrease in power resulting from turning off M of the N transistors is given by Equation 1. EQU Power Decrease (dB)=10 log ((N-M)/N).sup.2 ( 1)
Thus, for the case of two transistors one of which is turned off, the decrease in power is 6 dB, as described above.
For a network comprising only a small number of transistors, the amplitude control achieved by the conventional combining network is not very fine. For a network comprising a larger number of transistors, the amplitude control achieved by the conventional network is finer, but may still not be sufficient for the particular application. However, in all cases, RF power is wasted by dissipation in the RF loads at the isolated ports of the four port hybrid networks, in general requiring additional cooling.
Conventional combining networks operate in this manner because they have been designed to combine the outputs of a fixed number N of transistors with voltage coupling factors equal to 1/(N).sup.1/2 because each respective input port and the output port. When such a network is operated as a combiner of N-M transistors, the conventional combiner network cannot combine the reduced power without some loss.
One known amplitude controller comprises two cascaded 3 dB hybrid networks, with a PIN diode phase shifter coupling the two output ports of the first hybrid network (when viewed as a divider) to the input ports of the second hybrid. This device suffers from a relatively higher insertion loss and requires additional elements in addition to the two hybrid coupler networks.
Another known circuit employs a matched reactive Tee with PIN diodes. Transformers are employed for matching when both input ports are activated. When the PIN diodes are biased to a short circuit, one arm behaves as a shorted quarter wavelength stub and the other arm transforms the still active port to the incorrect impedance level for a match. Thus, this known device suffers from mismatch loss when one port is turned off.
It would therefore represent an advance in the art to provide an RF power combiner which not only achieves power combination but also finer amplitude control with less insertion loss than conventional combiner circuits.
It would also be advantageous to provide an RF power combiner which not only achieves power combination but also finer amplitude control, which employs relatively few elements and is of relatively small size.
It would also represent an advance in the art to provide an RF power combiner which not only achieves power combination but also finer amplitude control without dissipating, except for I.sup.2 R losses, the RF power the combiner was intended to combine.